The invention relates to a method to form a family of supported films exhibiting highly ordered microstructures and porosity derived from an ordered micellar or liquid-crystalline organic-inorganic precursor structure that forms during film deposition. Applications of these films include sensors, membranes, low dielectric constant interlayers, anti-reflective coatings, and optical hosts.
The International Union of Pure and Applied Chemistry, IUPAC, classifies porosity on the basis of pore diameter, d.sub.p. Mesoporous materials are defined by IUPAC as those materials in which 2 nm&lt;d.sub.p &lt;50 nm, although many other definitions abound in the literature. In keeping with prior art on this subject, we will define the mesoporous range as 0.8 nm&lt;d.sub.p &lt;20 nm.
Mesoporous inorganic materials comprise inorganic xerogels (e.g., the common silica desiccants), pillared clays, and the subject matter of this patent viz. mesoporous molecular sieves (MMS), recently discovered by researchers at Mobil (U.S. Pat. No. 5,098,684, issued to Kresge et al. On Mar. 24, 1992; U.S. Pat. No. 5,057,296 issued to J. S. Beck on Oct. 15, 1991) and referred to in the literature as the MCM (Mobil composition of matter) family of materials. MMS prepared generally as powders have received enormous attention by the research community since their announcement by Kresge et al. (Kresge C. T., Leonowicz M. E., Roth W. J., Vartuli J. C., Beck J. S., Nature, 1992, 359: 710-712). In the past two years, advances have been made in understanding and exploiting the supramolecular templating process used in MMS formation, development of new synthetic procedures, extending the compositional range beyond silicas, and processing of MMS as thin films. MMS are high surface area amorphous solids (surface area up to 1400 m.sup.2 /g) characterized by monosized cylindrical pores, ranging from about 12-100 .ANG. in diameter, organized into periodic arrays that often mimic the liquid crystalline phases exhibited by surfactants. MMS synthesis procedures typically require four reagents: water, surfactant, a soluble inorganic precursor, and catalyst. MMS form (as precipitates) in seconds to days (Beck J. S., Vartuli J. C., Roth W. J., Leonowicz M. E. Kresge C. T., Schmitt K. D., Chu C. T. W., Olson D. H., Sheppard E. W., McCullen S. B. et al., J. Am. Chem. Soc., 1992, 114: 10835; Huo Q., Margolese D. L., Ciesla U., Demuth D. G., Feng P., Gier T. E., Sieger P., Firouzi A., Chmelka B. F., Schuth F., Stucky G. D., Chem. Mater., 1994, 6: 1176-1191) at temperatures ranging from 180.degree. C. to as low as -14.degree. C., depending on the inorganic precursor. Before pyrolysis or surfactant extraction, pure silica MMS exhibit three structure types: (1) hexagonal (referred to as H or MCM-41), a 1-d system of hexagonally ordered cylindrical silica channels encasing cylindrical surfactant micellar assemblies; (2) cubic (C), a 3-d, bicontinuous system of silica and surfactant; and (3) lamellar, a 2-d system of silica sheets interleaved by surfactant bilayers.
Over the past several years various MMS synthetic pathways have been elucidated (Beck J. S., Vartuli J. C., Curr. Opinion in Solid State and Material Science, 1996, 1: 76-87). Experimentally, it has been shown that MCM-41 type phases form under conditions in which the surfactant--before the addition of the silica source--is: a) free (surfactant concentration, c, is less than the critical micelle concentration for spherical micelles, c&lt;c.sub.mc1), b) in the form of spherical micelles (c.sub.mc1 &lt;c&lt;c.sub.mc2, where c.sub.mc2 is the critical micelle concentration for cylindrical micelles ), and c) in the form of cylindrical micelles (c.sub.mc2 &lt;c&lt;c.sub.LC, where c.sub.LC, is the concentration for formation of the liquid crystalline (LC) phase), or d) in the form of liquid crystalline phases. These findings indicate that silica does not in general simply petrify a pre-existing LC array but rather cooperatively co-assembles with the surfactant to form LC phases during the course of the synthesis. Specific details of the co-assembly mechanism are still controversial. Three models have been postulated: a puckering layered model, silicate rod assembly, and cooperative charge density matching. Regardless of the specifics, it has been shown that silica condensation is not essential to the assembly process. Using anionic cubic octamers Si.sub.8 O.sub.20.sup.8-, Firouzi et al. (Firouzi A., Kumar D., Bull L. M., Besier T., Sieger P., Huo, Q., Walker, S. A., Zasadzinski J. A., Glinka C., Nicol J., Margolese D., Stucky G. D., Chmelka B. F., Science, 1995, 267 1138-1143) demonstrated reversible lamellar to hexagonal phase transformations. Nor are electrostatic interactions essential. Tanev and Pinnavaia (Tanev P. T., Pinnavaia T. J., Science, 1995, 267: 865-867) and Bagshaw et al. (Bagshaw A. S., Prouzet E., Pinnavaia T. J., Science, 1995, 269: 1242-1244) have demonstrated the formation of MMS using two neutral routes based on hydrogen bonding and self-assembly between nonionic primary amine or polyethylene oxide surfactants and neutral oligomeric silica precursors. Tanev and Pinnavaia (Tanev P. T., Pinnavaia T. J., Chem. Mater., 1996, 8: 2068-2079) compared ionic and neutral surfactant-templated MMS and concluded that, although the ionic surfactant-templated MMS have in general greater order, the neutral surfactant templated MMS has thicker walls, a greater extent of condensation, improved thermal stability, and greater textural mesoporosity. In addition, the combination of a neutral framework and extensive condensation permit template removal by solvent extraction.
In the past several years, there has been synthesized multicomponent and non-silica MMS (Huo Q., Margolese D. L., Ciesla U., Demuth D. G., Feng P., Gier T. E., Sieger P., Firouzi A., Chmelka B. F., Schuth F., Stucky G. D., Chem. Mater., 1994, 6: 1176-1191) for catalytic applications due to their higher surface areas and greater accessibility of active sites compared to zeolites. Particular attention has been paid to titanium incorporation in silica MMS based on expectations that, by analogy to the ability of TS-1 (Ti-doped high silica ZSM-5) to selectively oxidize alkanes, alkenes, and alcohols, titania-silica MMS (Ti-MMS) may perform shape selective oxidation of larger organic molecules. Ti-MMS have been synthesized by incorporation of titanium into the silica framework or by grafting titanocene complexes on pre-formed silica MMS, the latter procedure providing higher surface concentrations and accessibility of catalytic sites. Oxidation studies have confirmed the ability of Ti-MMS to perform large molecule oxidation. For example, selective epoxidation of norborene using tertbutylhydroperoxide as an oxidant could be achieved with Ti-MMS but not TS-1 due to the bulky nature of the reactants. Additionally, shape selective conversion of 2,6-di-tert-butyl phenol has been demonstrated along with enhanced activity when Ti is grafted as a pendant site on the pore interior. However, when comparing oxidation reactions of small molecules such as the epoxidation of hexene by H.sub.2 O.sub.2, TS-1 exhibits much higher activity than Ti-MMS, and reactions that occur readily with TS-1, such as primary amine oxidation, practically do not occur over Ti-MMS. The reasons for these differences in reactivity are presently unclear but altered framework crystallinity, hydrophilicity, and Ti redox potential may be contributing factors.
With regard to non-silica frameworks and hybrid structures, several recent reports are noteworthy. Antonelli and Ying (Antonelli D. M., Ying J. Y., Angew. Chem. Int. Ed. Engl., 1996, 35: 426-430; Antonelli D. M., Ying J. Y., Chem. Mater., 1996, 8: 874-881) developed a ligand-assisted templating scheme to prepare pure niobium and tantalum oxide MMS that were stable to surfactant removal and hydrothermally stable to temperatures ranging from 300.degree.-800.degree. C. Neutral primary amine surfactants were pre-reacted with metal alkoxides to form new metal organic surfactant molecules which were hydrolyzed in a second step to produce pure Nb or Ta MMS. The surfactant was removed by acid washes that cleaved the metal-nitrogen bond. Ciesla et al. (Ciesla U., Schacht S., Stucky G. D., Unger K. K., Schuth F., Angew Chem Int. Ed. Engl., 1996, 35: 541-543) developed phosphate and sulfate complexation schemes to stabilize zirconium oxide MMS. Phosphate and sulfate ions assisted in the formation of zirconia-surfactant composites and promoted further crosslinking of the zirconium oxo species, stabilizing the porous structure to 500.degree. C. Using tetraalkoxysilane and substituted organoalkoxysilane precursors, Burkett et al. (Burkett S. L., Sims S. D., Mann S., Chem. Comm. 1996, 1367-1368) prepared hybrid inorganic-organic MMS containing covalent Si-phenyl and Si-n-octyl bonds. Kloetstra et al. (Kloetstra K. R., Jansen J. C., van Bekkum H., Proc. of Symp. on Advances in FCC Conversion Catalysts, 211th National Meeting, Div Petrol Chem, Am Chem Soc Nat Mtg, New Orleans, La. Mar. 24-29, 1996) hydrothermally treated calcined, aluminosilicate MMS with tetrapropylammonium hydroxide and formed Al--Si MMS containing zeolitic ZSM-5 micro-domains. These composite materials were shown to be active for hexane cracking under conditions where commercial aluminosilicate catalysts and the parent Al--Si MMS were not.
Films exhibiting a unimodal pore size are frequently cited as a promising application of mesoporous materials, and very recently Yang et al. (Yang H., Coombs N., Sokolov I., Ozin G. A., Nature, 1996, 381: 589-592) showed that by exceeding the critical micelle concentration c.sub.mc of a bulk silica-surfactant solution, hexagonal liquid crystalline silica mesophases formed at solid-liquid and liquid-vapor interfaces. It was proposed that the growth process involved self-assembly of silica-surfactant micellar species on interfacially organized structures. However this process is relatively slow, e.g. .about.0.2-1.0-.mu.m thick films were grown over a period of 1 h -1 week at 80.degree. C., and results in opaque, granular films that appear inhomogeneous on the several-.mu.m length scale. None of these previous studies have demonstrated accessibility of the mesophase porosity of a supported film.